PASADENA, Calif. (AP) -- A NASA spacecraft with a Hollywood name released a probe early Sunday on a collision course with a speeding comet, an ambitious mission that scientists hope will offer the first peek inside one of these icy bodies.

Deep Impact released its barrel-sized "impactor" at 2:07 a.m. EDT on a suicide journey that is expected to climax 24 hours later when the comet Tempel 1 smashes into it. The high-speed crash will produce an Independence Day weekend explosion that should be visible from parts of the Western Hemisphere.

Comets contain the frozen primordial ingredients of the solar system and studying them could provide clues to how the sun and planets formed.

NASA says an impact will not significantly change the comet's orbital path around the sun, so the $333 million experiment poses no danger to Earth.

The 820-pound copper probe successfully separated from the mothership to set the stage for the collision with the comet, according to mission control at NASA's Jet Propulsion Laboratory in Pasadena. Electrical wires connecting the spacecraft broke, springing free the probe.

Scientists are counting on the collision to carve a stadium-sized crater in Tempel 1, a pickle-shaped comet half the size of Manhattan now about 80 million miles from Earth. No explosives are needed since the energy from the impact will be similar to detonating nearly 5 tons of TNT.

It is the first attempt by the U.S. space agency to catch a glimpse of the pristine core of a comet.

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After its release, the battery-powered probe began a 500,000-mile plunge toward the sunlit side of Tempel 1. Meanwhile, the mothership must fire its thrusters to slightly change course and stake out a front-row seat 5,000 miles from the collision, which is expected to occur around 10:52 p.m. PDT Sunday.

The probe will switch to autopilot two hours before Sunday night's encounter, relying on computer software and thrusters to steer itself into the path of the onrushing comet. If the probe's maneuvers are off, the comet could miss and the mission would fail...